1. Field of the Invention
The present invention relates to a multivibrator circuit and, more particularly, to a multivibrator circuit employing field effect devices for outputting an output signal of a high oscillation frequency.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing a emitter-coupled multivibrator circuit employing conventional silicon npn type bipolar transistors. A similar circuit is described in "Bipolar and MOS Analog Integrated circuit Design" by A. B. Grebene, published by Micro-Linear Corporation in 1984.
Referring to FIG. 1, the multivibrator circuit comprises four series connections between a power supply V.sub.cc and the ground V.sub.ee. The first series connection comprises a first parallel connection of a resistor R.sub.11 and a diode D.sub.11, and npn type bipolar transistors Q.sub.11 and Q.sub.13. The emitter of the transistor Q.sub.11 and the collector of the transistor Q.sub.13 are connected together to constitute a node N.sub.13. The second series connection comprises npn type bipolar transistors Q.sub.15 and Q.sub.17. The emitter of the transistor Q.sub.15 and the collector of the transistor Q.sub.17 are connected together to constitute a node N.sub.11. The node N.sub.11 is connected to a first output OUT.sub.1. The base of the transistor Q.sub.15 is connected to a connection point between the collector of the transistor Q.sub.11 and the first parallel connection. The third series connection comprises a second parallel connection of a resistor R.sub.12 and a diode D.sub.12, and npn type bipolar transistors Q.sub.12 and Q.sub.14. The emitter of the transistor Q.sub.12 and the collector of the transistor Q.sub.14 are connected together to constitute a node N.sub.14. The base of the transistor Q.sub.12 is connected to the first output OUT.sub.1. A capacitor C.sub.11 is connected between the node N.sub.13 and the node N.sub.14. The bases of the transistors Q.sub.13 and Q.sub.14 are connected together and a connection point therebetween is connected to a first constant voltage supply V.sub.co. The fourth series connection comprises npn type bipolar transistors Q.sub.16 and Q.sub.18. The emitter of the transistor Q.sub.16 and the collector of the transistor Q.sub.18 are connected together to constitute a node N.sub.12. The node N.sub.12 is connected to a second output OUT.sub.2. The base of the transistor Q.sub.16 is connected to a connection point between the collector of the transistor Q.sub.12 and the second parallel connection. The base of the transistor Q.sub.11 is connected to the second output OUT.sub.2. The bases of the transistors Q.sub.17 and Q.sub.18 are connected together and the connection point therebetween is connected to a second constant supply voltage V.sub.CB.
Next, description is made of operation of the circuit.
Either one of the transistors Q.sub.11 and Q.sub.12 constituting an emitter-coupled switch is always on and the other is off.
First, assuming that the transistor Q.sub.11 is off and the transistor Q.sub.12 is on, a current I flows from the node N.sub.14 to the node N.sub.13 through the capacitor C.sub.11 because the current I is always flowing through the transistors Q.sub.13 and Q.sub.14 serving as constant current load in response to a voltage applied to a first control input. Therefore, a current of 2I flows through the transistor Q.sub.12. A collector voltage of the transistor Q.sub.12 is decreased by a voltage of 2IR.sub.2 by a resistor R.sub.12 (a resistance value is also represented by R.sub.12). Since a barrier voltage .phi..sub.B of the diode D.sub.12 and a value (R.sub.12) of the resistor R.sub.12 are selected so as to satisfy the relation of 2IR.sub.12 .gtoreq..phi..sub.B, the collector voltage of the transistor Q.sub.12 is clamped at V.sub.cc -.phi..sub.B. As a result, an emitter voltage V (N.sub.12) of the transistor Q.sub. 16 becomes V.sub.cc- 2.phi..sub.B which is lower than a base voltage V.sub.cc -.phi..sub.B by the voltage .phi..sub.B.
On the other hand, when an emitter voltage V (N.sub.13) of the transistor Q.sub.11 is reduced by the current I flowing through the capacitor C.sub.11 and becomes V.sub.cc- 3.phi..sub.B which is lower than the base voltage V.sub.cc -2.phi..sub.B of the transistor Q.sub.11 by the voltage .phi..sub.B, the transistor Q.sub.11 is turned on. As a result, a base voltage of the transistor Q.sub.15 is reduced from V.sub.cc to V.sub.cc -.phi..sub.B and an emitter voltage V (N.sub.11) of the transistor Q.sub.15 is reduced from V.sub.cc -.phi..sub.B to V.sub.cc- 2.phi..sub.B. Since an emitter voltage V (N.sub.14) of the transistor Q.sub.12 changes with a delay of a sufficiently large time constant by the capacitor C.sub.11 as compared with switching operation of the transistors Q.sub.11 and Q.sub.12, it is equal to an emitter voltage of V.sub.cc- 2.phi..sub.B when the transistor Q.sub.12 is on. Therefore, the transistor Q.sub.12 is turned off.
After the transistor Q.sub.12 is turned off, the base voltage of the transistor Q.sub.11 is raised by a voltage .phi..sub.B and the voltages V (N.sub.13) and V (N.sub.14) across the capacitor C.sub.11 are raised from V.sub.cc- 3.phi..sub.B to V.sub.cc- 2.phi..sub.B and from V.sub.cc- 2.phi..sub.B to V.sub.cc -.phi..sub.B, respectively. As a result, the states of the transistors Q.sub.11 and Q.sub.12 initially set have been changed with each other.
FIG. 2 is a timing chart showing changes of the first and second output voltages V (N.sub.11) and V (N.sub.12) of the emitter-coupled multivibrator circuit in FIG. 1 and the voltages V (N.sub.14) and V (N.sub.13) of the nodes N.sub.14 and N.sub.13 as well as a change of the voltage difference V (N.sub.14)-V (N.sub.13) across the capacitor C.sub.11.
An oscillation frequency f.sub.0 of the multivibrator circuit shown in FIG. 1 is represented by the following equation; ##EQU1## where C is a capacitance value of the capacitor C.sub.11, I is a current flowing through the transistors Q.sub.13 and Q.sub.14 for constant current loads of the emitter-coupled switch portion, and reference 2.phi..sub.B is a maximum amplitude voltage applied across the capacitor C.sub.11. Since the current I can be controlled by a voltage V.sub.co applied to the bases of the transistors Q.sub.13 and Q.sub.14, the emitter-coupled multivibrator circuit shown in FIG. 1 is a voltage-controlled type oscillation circuit.
Since the conventional emitter-coupled multivibrator circuit shown in FIG. 1 employs a silicon bipolar transistor, it involves disadvantages such as a low oscillation frequency and a large power consumption.